\begin{answer}{arraymissingnumber}
This is question 5.4 from \citet{JoshiQA}.
For the case of one missing number, you know the sum is supposed to be
\[
  \frac{n (n+1) }{2}
  \text{,}
\]
so you can calculate the sum of the $n$ numbers, and the difference will be the missing number.
The computational complexity of this is $O(n)$, since you need to add the $n-1$ numbers.
\end{answer}